Pilot-assisted air vehicles, or even unmanned air vehicles or UAVs, also known as “drones”, presently fly in segregated zones. However, ATC (air traffic control) airspace, in which notably civil aircraft fly, is set to become populated by an increasing number of vehicles of the aforementioned types. It is consequently necessary for these aircraft to be able to fly in all classes of air space, and to cross frontiers, without them in any way representing a risk to humans. Thus, these aircraft must demonstrate a level of safety at least equal to that of aircraft having a pilot on board, whether they are being flown by the pilot or are preprogrammed or indeed operating completely automatically. Drones must notably employ a sense-and-avoid system enabling them to detect objects that may potentially be obstacles to their flight and to implement avoidance procedures where appropriate. A sense-and-avoid system relies on a detection and tracking “sense” function and on an “avoid” function, which must pre-emptively modify the path of the aircraft in a protected zone, typically defined by a sphere centred on the aircraft, with a radius of 500 feet. This avoid function, in the case of cooperative equipment, is effected by the TCAS (the acronym for Traffic Alert Collision Avoidance System) or by the T2CAS (the acronym for Terrain and Traffic Collision Avoidance System) if detection of the ground is integrated by coupling with the data delivered by a radioaltimeter with which the aircraft is equipped.
A sense-and-avoid system may present a risk with respect to the safety of personnel and must consequently meet stringent requirements in terms of reliability and effectiveness. Such requirements are specified in standards, for example in the NATO standard STANAG 4671. Notably, it is necessary for a sense-and-avoid system to have an angular coverage at least equivalent to the visual coverage of a human pilot, i.e. typically about ±110° in azimuth and about ±20° in elevation. It is also required that the system be effective whatever the weather conditions. All these requirements are itemized in airborne radar system specifications and notably determine therein the angular performance characteristics, and also the range, that have to take into consideration the data refresh time. These requirements notably mean that a compromise has to be chosen between the range and the data refresh time, over a very wide angular field.
Radar systems fitted in aircraft known in the prior art notably comprise at least one mechanically rotated or electronic scanning antenna. Such systems operate at a high frequency and consequently have a narrow beam. They thus allow precise tracking, but their scanning rate must however be extremely high so as to cover the required wide angular field sufficiently rapidly. Furthermore, such systems have the drawback of requiring an excrescence on the structure of the drone, accommodating the motorized or electronic scanning antenna structure. This drawback entails aerodynamic and/or size constraints. In addition, the structure of the aircraft systematically incorporates components such as a landing gear, wings, etc., which represent as many masks, imposing an almost unique position of the antenna structure. This position is usually located on the nose of the aircraft, which is that part most exposed to impacts, for example by birds. Since the antenna structure is centred thereat, an impact on the nose of the aircraft may then entail a complete loss of the sense-and-avoid function. The redundancy of such a system is also tricky, if not impossible, to realize in practice.
Another drawback of the known systems of the prior art lies in the fact that they do not allow a plurality of functions to be carried out on the basis of the same physical architecture. Notably, the radar segment of systems intended for carrying out the sense-and-avoid function in drones operates in the millimeter band, typically in the Ka-band or the Ku-band. Now, the Ka-band for example does not enable weather conditions to be detected, narrow-beam or scanning radars not being able to carry out more than one function simultaneously. Thus, a scanning radar does not allow for a weather radar, itself operating in scanning mode, to be reliably detected since the probability of intercepting the signals is low.
Another drawback of the known systems of the prior art also lies in the fact that antenna scanning entails a relatively low probability of detecting obstacles. Moreover, the known systems of the prior art cannot hierarchize the danger level of the detected targets. In these systems, the target tracking can be carried out only in sampled mode, with the consequence of there being a risk of confusion, mainly in the presence of ground clutter, or else of ground vehicles. These systems focus onto particular targets, by switching from a standby mode to a tracking mode, but such switching nevertheless impairs the detection of potentially hazardous new targets.
Thus, multi-target tracking is limited by the mechanical constraints on the antenna. Such tracking may be implemented at very high scanning rates, for example with scanning times of less than two seconds, but this means a short integration time on the target and requires the use of high transmission power levels. Consequently, in the systems known from the prior art, since the refresh times are long and the integration time on the target is short, it is not possible for the receive channel to alleviate the deficiencies associated with scanning. The transmission power levels necessary for accomplishing the sense-and-avoid function are therefore high, with the following drawbacks:                high power consumption;        the necessity of operating the radar in pulsed mode;        the necessity of using higher operating frequencies, for the purpose of allowing satisfactory spatial segregation, to the detriment of efficiency;        the difficulty of ensuring effective heat dissipation, since the transmitter is centralized;        the necessity of typically dedicating the nose of the aircraft to implementing the sense-and-avoid function;        the difficulty of providing satisfactory immunity to electromagnetic radiation, because of the high transmission power;        the difficulty of making a system based on moving components of major criticality reliable;        antenna scanning has the consequence that targets are tracked discontinuously, these being sampled at the scanning rate. It is also necessary to associate, with each scan, the detected echoes with the corresponding tracks, even during turns by the carrier, except if the inertial guidance system of the latter is integrated into the tracking system, the inertial guidance system then being critical for the sense-and-avoid function;        the limitations inherent in multi-target tracking, notably the limitation on the number of tracks followed, and the necessity of operating specific antenna pointing means; and        the necessity for a long illumination time for targets having a low radar cross section or RCS, making it difficult to achieve a good compromise between the data refresh rate and the quality of the tracking.        
Known systems of the prior art for providing a sense-and-avoid function may also be based on cooperative modes; however such systems have the following drawbacks:                small private planes, powered ultra-lights, delta wings and balloon probes are not equipped with cooperative means; and        in dense traffic zones, the existing standards relating to cooperative modes impose avoidance procedures on aircraft by a change of altitude, yet aircraft of the drone type may be incapable of sudden changes of altitude, because of a lack of engine power and because their aerodynamic finesse does not allow them to ascend or descend rapidly.        
In any case, and for the reasons related to the aforementioned drawbacks, the use of cooperative modes alone is not feasible for providing a sense-and-avoid function.